Email preview may have different meanings in email marketing context.
At recipient level email preview refer to the fact that some email clients allow recipients to see the first lines or part of an email in the main window or in a preview reading pane without opening the message.
Email preview by recipients may slightly affect opening measurement adn may be taken into account for email designing.
Email previewing is also used by email marketers / designers when designing an HTML email for seeing how the code is rendering. Email preview may be done directly where the HTML is created or pasted, in this case it is a simple preview.
In other cases, email preview is a specific service or tool that allows to control email rendering in many email clients and configurations. For more details see email rendering test.